1. Field of the Invention
This invention relates to data modulators/demodulators (modems) and in particular to an improved data modem for use on communication channels.
2. Description of Prior Art
Time-varying or fading communications channels seriously hamper high speed digital communications. Examples of fading communications channels are high frequency (HF) ionospheric scatter, meteor-burst, two-way radio, cellular radio, microwave radio, mobile satellite, and underwater acoustic channels. The fading channel requires many orders of magnitude greater average receive power to achieve the same performance as the equivalent non-fading channel. Consequently, the common practice on fading channels is to operate with a large power margin or fade margin to ensure reliable communications. On microwave radio channels, it is common to design a 40 decibel (dB) (factor of 10,000) fade margin on each link. If additional power is not available, prior art systems must operate at extremely low data rates to ensure reliable communications.